The present invention generally relates to power amplifier current modulation, and particularly relates to inferentially sensing changes in power amplifier resistance.
Various techniques exist for generating radio frequency transmit signals used in wireless communication systems. These various approaches each entail advantages and disadvantages, but one consideration that is of significant importance with regard to mobile communication devices is the need for relatively good operating efficiency. Operating efficiency is generally a significant factor in the battery life of mobile communication devices. Power amplifier efficiency is a primary factor in overall operating efficiency because the power amplifier circuits used to generate radio frequency (RF) transmit signals are primary consumers of power in such devices.
One technique used to gain power amplifier efficiency involves polar modulation where phase and amplitude modulation information representing desired transmit data are separated into an amplitude modulation signal and a phase modulation signal. The phase modulation signal is generated as a constant-envelope signal applied to the amplification input of the power amplifier. The power amplifier is operated in saturated mode, which yields significant gains in operating efficiency but prevents linear amplification of variable amplitude input signals. To impart linear amplitude modulation to the output signal from the power amplifier, its supply voltage or supply current is modulated in accordance with the desired amplitude modulation information. This yields a time varying RF output signal from the amplifier having both phase and amplitude modulation information.
Supply voltage modulation is relatively straightforward, and involves the use of a voltage modulator to modulate amplifier supply voltage responsive to a controlling amplitude modulation signal. While relatively straightforward, supply voltage modulation is sometimes undesirable where power amplifier non-linearity is encountered due to, for example, transistor turn-on voltage offsets.
Supply current modulation offers superior linearity but is not without its attendant challenges. For example, the power amplifier has low frequency xe2x80x9cAM modulation impedancexe2x80x9d such that supply current modulations produce a resultant power amplifier operating voltage that swings within a nominal operating range. However, the actual amplifier impedance (resistance) seen by the modulated supply current varies as a function of its RF output impedance coupling. Thus, as the coupling characteristics of the transmit antenna change, the operating voltage resulting from the modulated supply current can vary dramatically. As the affected AM modulation impedance of the power amplifier increases, its operating voltage may rise up to supply limits, beyond which the RF output signal is clipped. Such clipping causes potentially significant non-linearity resulting in a host of undesirable effects including increased adjacent channel signal interference and higher communication bit error rates.
While some conventional systems employing current modulation attempt to compensate for power amplifier impedance changes, existing approaches rely on the use of dissipative sense elements, such as transmission line couplers, to detect RF output signal power. These existing approaches waste transmit signal power, and therefore reduce operating efficiency.
The present invention provides a method and apparatus to dynamically compensate for changing power amplifier impedance in conjunction with providing amplitude modulated supply current to the power amplifier. This dynamic compensation is based on inferentially sensing changes in the DC resistance of the power amplifier arising from changes in power amplifier impedance associated with, for example, changes in transmit antenna coupling. Changes in power amplifier AM modulation impedance, such as changes in power amplifier output impedance, are inferred from the power amplifier operating voltage and a reference current maintained in fixed proportion to the modulated supply current into the power amplifier. This approach yields accurate sensing of changes in power amplifier impedance but without the need for using dissipative components, such as directional couplers, in either the current supply path or RF output signal path of the power amplifier.
A current modulator according to some embodiments of the present invention inferentially senses power amplifier output impedance (PA resistance) to dynamically adjust a control gain used in generating the scaled reference current to maintain a fixed overall amplitude modulation gain between an amplitude modulation signal and the resultant modulated supply current. With this approach, modulation gain may be maintained over a relatively wide range of changing PA resistance. An exemplary approach used in these embodiments involves varying a gain control resistance within a modulation control feedback loop responsive to inferentially sensed PA resistance.
Alternatively, gain control may be used to vary the gain responsive to changing PA resistance to avoid voltage clipping arising from driving the modulated supply current into an increasing PA resistance. That is, the modulation gain may be reduced as the PA resistance increases to avoid driving the operating voltage of the power amplifier to the voltage limits of the current modulator. Gain control may involve the use of a variable resistance, or may involve translating the inferentially sensed PA resistance into adjustments of the modulation signal driving the current modulator. Such adjustments might be made in the analog or digital domains, or in both.
In still other embodiments, matching network impedance is adjusted responsive to the inferentially sensed PA resistance to compensate for changes in transmit antenna coupling. In these configurations, the RF output signal from the power amplifier couples to the transmit antenna through a variable matching network that can be adjusted to maintain impedance matching over a range of impedances. By maintaining impedance matching between the RF output and the transmit antenna, reflected RF power is maintained at a nominal voltage standing wave ratio (VSWR) value, and the current modulator providing the modulated supply current xe2x80x9cseesxe2x80x9d an essentially constant PA resistance looking into the supply input of the power amplifier.